RADIOACTIVITY
LIBRARY: Find out the following:
How does a smoke detector work?
How does a Geiger tube (also known
as “Geiger-Müller” tube) work? What do
the following terms mean in relation to it: “voltage plateau”,
“avalanche region”?
What are the isotopes of americium?
What are their half-lifes? What does each one emit as
it decays?
What is the “dead time”
of a radiation detector? How does one use a “split source” to
measure dead time? Find an equation.
DETECTORS, STOPPING POWER:
Set up both a Geiger-Müller tube and a
computer-interfaced “microRoentgen”
radiation detector to measure the radioactivity of the americium source inside
a smoke detector. Use the appropriate lab hardware to place either detector an
appropriate distance above the source. Be sure to adequately cover the smoke
detector assembly when you are done today.
For the Geiger tube, measure the count rate as a function of
Voltage. Start at the lowest Voltage for which you get counts, and increase
Voltage until you reach the “avalanche” regime. Do not exceed 1000V.
For both detectors, measure the maximum count rate you can
get from the source. Does inserting one, two, three or more pieces of paper
between the source and the detector affect the count rate? How about various
identical sheets of other materials? Comment on your results. What can you
conclude about the type[s] of radiation emitted by the americium?
INVERSE-SQUARE LAW:
The activity a detector records for some source might be expected to fall off
as the inverse square of the distance between the detector and the source of
the radiation:
Count = A/r2.
Verify this. Notice that you will not be able to measure the
distance between the source and the detector accurately. (which
part of the source? which part of the detector?) A better way to
check whether there is an inverse square law is to test whether
Count-1/2 = rA-1/2.
Do your data fit a straight line? Calculate A and x from
your fit, if such a fit is appropriate. Interpret your results.
DEAD TIME:
Some radiation detectors, like a Geiger tube, suffer from 'dead time'. After an
event has triggered the tube, it is 'dead', or unresponsive, for some time
thereafter, and cannot respond to further events. There are three ways you can
quantify the dead time, and you should do each for the GM tube. (1) From the
maximum count rate. The dead time must be shorter than 1 divided by this count
rate. Find an upper bound. (2) Using the oscilloscope. Look at the output of
the GM tube “scaler” on the scope. By
inspection, you can get a rough measure of the dead time. Be sure to use the
storage scope to get a "time lapse" printout of the detector signal.
(3) Using a split source. Look up how to calculate dead time for a split
source, then take very careful data. You need to
calculate the uncertainty of your measurement of dead time.
When you have finished, compare your three different
measurements of dead time. Be sure to comment on whether they are consistent,
given your uncertainties.
HALF-LIFE, DECAY RATE, AND AMOUNT OF RADIOACTIVE
MATERIAL:
From purely practical considerations, determine which isotope(s) of americium
are likely to be used in your smoke detector. Measure the activity of the
americium (making sure to subtract the background level). Check this against
the activity marked on the smoke detector itself and calculate the efficiency
of your setup in counting radiation. Is the dead time of your detector to blame
for any low efficiency? Estimate the total mass of americium in the source,
(using the activity marked on the source). You may need to differentiate the
expression for the number of remaining radioactive nuclei as a function of time
to get this relation.
HALF-LIFE. Time permitting,
we will analyze some muon decay data taken at St.
Lawrence to calculate the half-life of muons.